Our hypotheses are nitric oxide (NO) will contribute less to cutaneous vasodilation during both whole body heating and local heating in older versus young subjects; and in post-menopausal women estrogen replacement therapy will augment the contribution of NO-mediated vasodilation during both whole body and local heating. The objective of this research project is to investigate the effects of healthy aging on the physiologic mechanisms that participate in the regulation of skin blood flow in humans. The specific aims are to study the nitric oxide-mediated dilation of blood vessels in non-acral skin of young and older individuals; to determine if the reduced active vasodilator response with aging during local heating is due to a structural limitation (smooth muscle) or a deficit in the vasodilator signal; and to determine if estrogen hormone replacement therapy in post-menopausal women augments cutaneous vasodilation.